Solar energy, atomic power, wind power, geothermal heat, re-utilization of waste heat, etc, have been proposed as new energy sources to replace fossil fuel from the aspect of the environmental problems of the earth. For all of these sources, a common problem is how to store and transport energy. A system which electrolyzes water by using solar energy or water power and uses the resulting hydrogen as an energy medium can be said to provide ultimate clean energy in the sense that the starting material is water and the product obtained by consuming this energy is also water.
As one of the means for storing and transporting this hydrogen, a hydrogen-absorbing alloy can absorb and store a hydrogen gas to a capacity about 1,000 times the volume of the alloy itself, and its volume density is substantially equal to, or greater than, that of liquid or solid hydrogen. It has long been known that metals and alloys having a body-centered cubic lattice structure (hereinafter called the "BCC structure"), such as V, Nb, Ta, Ti--V alloys, etc, absorb and store greater amounts of hydrogen than an AB.sub.5 type alloys such as LaNi.sub.5 and AB.sub.2 type alloys such as TiMn.sub.2 that have been already put into practical application. This is because the number of hydrogen absorbing sites in the crystal lattice is large in the BCC structure, and the hydrogen absorbing capacity according to calculation is as great as H/M=2.0 (about 4.0 wt % in alloys of Ti or V having an atomic weight of about 50).
A pure vanadium alloy absorbs and stores about 4.0 wt %, which is substantially similar to the value calculated from the crystal structure, and desorbs about half this amount at normal pressure and room temperature. It is known that Nb and Ta as the elements of the same Group 5A of the Periodic Table exhibit a large hydrogen storage capacity and excellent hydrogen desorption characteristics in the same way as vanadium.
Because pure V, Nb, Ta, etc, are extremely high in cost, however, the use of these elements is not realistic in industrial application which requires a considerable amount of the alloys, such as a hydrogen tank or a Ni--MH cell. Therefore, properties of alloys have been examined within the range having the BCC structure such as Ti--V, but new problems have arisen in that these BCC alloys merely absorb and store hydrogen at a practical temperature and pressure but that their hydrogen desoprtion amount is small, in addition to the problems encountered in V, Nb and Ta in that the reaction rate is low and activation is difficult. As a result, alloys having a BCC phase as the principal constituent phase have not yet been put into practical application.
The conventional attempt to control the characteristics by alloying has been carried out by component design in all of the AB.sub.5 type, the AB.sub.2 type and the BCC type. However, the set range of the components does not exceed the category of the inter-metallic compound single-phase and the BCC solid solution single-phase in all of these examples. As one of the known references in this field, Japanese Unexamined Patent Publication (Kokai) No. 7-252560 discloses an alloy which has a composition of five or more elements, has a body-centered cubic structure as a crystal structure, comprises a Ti--Cr system as the basic system and is expressed by the general formula Ti.sub.100-x-y-z Cr.sub.x A.sub.y B.sub.z, where A is at least one member selected from the group consisting of V, Nb, Mo, Ta and W, and B is at least one member selected from the group consisting of Zr, Mn, Fe, Co, Ni and Cu. As the effects of the alloy, this reference describes that a sufficient hydrogen absorption effect cannot be expected by optimization of the lattice constant alone, and when the size of virtual spheres that can be arranged in spacing is at least 0.33 .ANG. in terms of their radius, the hydrogen absorption amount increases drastically. However, this reference does not have the concept of utilizing the region in which the spinodal decomposition occurs, but only stipulates the lattice constant.
The quaternary alloy system having the BCC structure according to the prior art described above are arranged by handling the solid solution as a single phase, though the metallic structure is a multi-phase. In other words, all of the prior art references do not pay specific attention to the metallic structure of the alloys having two or more phases, do not either mention to control the metallic structure, and do not at all describe the phases other than the single phase. As to the effect, too, the prior art references mitigate the reaction rate and the activation condition to a certain extent but do not succeed in improving the desorption characteristics themselves, that is, the mitigation of the absorption and desorption temperature and the pressure condition. In this way, creating the technology of achieving the multi-phase structure by understanding the influences of the multi-phase and controlling this multi-phase structure so as to drastically increase the capacity and to mitigate the absorption and desorption characteristics has not yet been accomplished. Therefore, the technical development of a hydrogen-absorbing alloy capable of further improving these characteristics by the multi-phase control technology has therefore been desired.